Before starting Strava Group, I spent seven years on the tools as a carpenter and five years running sites across Sydney. In that time I saw inside hundreds of renovated and extended homes, and the failures I kept seeing were never random. They were in the same place every time: the junction, where new work meets the original house. The cracked cornice tracing the old wall line. The roof that leaks exactly where new metal meets old tiles. The floor that steps where two slabs settled differently. This post is about why that happens, and what a builder has to actually do to stop it.

Why the junction is where it goes wrong

An extension bolts a new structure onto an old one, and the two want to behave differently. The original house finished settling decades ago. The new work is on fresh footings in soil that moves with moisture, especially the clay soils across the Hills, Parramatta and most of the inner west. New timber shrinks as it dries; hundred year old hardwood does not. New concrete cures and creeps; the old slab is done moving. Two structures, two behaviours, one join. If that join is not designed and detailed for movement, the building tells you within a few years, in cracks, leaks and steps.

Here is the part that should bother you: almost none of these failures break any rule. A build can pass every inspection and still crack along the junction in year three, because the minimum standard is about safety, not longevity. The difference between a junction that fails and one that lasts is not compliance. It is detailing, and detailing is a choice the builder makes.

The five failures we see, and what prevents each one

1. Cracking along the old wall line. Caused by new structure moving against old with nothing but plaster bridging the two. Prevented by a proper structural tie between the frames and a movement detail where they meet, so the join is designed to flex where you cannot see it instead of cracking where you can.

2. Leaks where new roof meets old. Caused by flashing treated as an afterthought, silicon doing a job metal should do. Prevented by the new-to-old roof junction being flashed and detailed like the wet area it is, inspected and photographed before it is covered.

3. Stepped and bouncy floors at the join. Caused by new footings settling at a different rate to the originals, or new floor levels never truly matched to old suspended floors. Prevented by footings engineered for the actual soil and detailed against the existing ones, and floor levels set off the real house, not the drawings.

4. Visible ceiling and wall joins between old and new rooms. Caused by sheeting new plasterboard straight onto two structures that will never sit in one plane. Prevented by furring channels run across old and new, giving one dead flat plane so you cannot see where the house ends and the extension begins.

5. The failing structure that gets covered instead of fixed. Caused by builders sheeting over tired framing at the junction because fixing it is not in the quote. Prevented by rectifying the adjoining structure, screwing off and re-bracing so new and old move as one building. Covering a problem is not repairing it, and paint is not a structural material.

How to use this before you sign with anyone

Every builder’s photos look the same, because photos show finished rooms and none of this shows up in a finished room. All of it shows up in year seven, when the builder is long gone. So ask the questions the photos cannot answer. How do you detail the junction between new and existing? How are the new footings designed against the old ones on my soil? What happens to the existing structure where the extension ties in? Is waterproofing a documented hold point, and do I get photos of everything before it is covered? A builder who does this work has specific answers and will enjoy the question. A builder who goes vague has told you everything.

What we do about it

On every extension, renovation and second storey addition we build, the junction detailing above is standard, not optional: structural ties and movement details at every new-to-existing join, footings engineered for the site, the adjoining structure rectified rather than covered, furring channels for one flat plane, waterproofing as a documented hold point, and a photographic record of everything before it is covered, handed to you at completion. None of it is visible in the finished photos. All of it is the difference between a build that looks good at handover and one that is still right in year twenty.

If you are planning work on your home and want it built to last rather than built to pass, tell us about your project and we will give you honest advice and real numbers, no cost, no obligation.